Radio Station FIX , SE-400 , original US designation SCR-608 ; manufactured by div. U.S. manufacturers.
To renew the equipment of the Swiss signal troops after the ye ars of World War II, in 1946/49 a large amount of signal equipment was purchased from American surplus stocks.
The U.S. station SCR-608 was acquired in this context; with this set, the VHF range was for the first time in Switzerland used for military communication. At the same moment, frequency modulation was also used for the first time in a military radio. The device got the designation „FIX“ resp. after the introduction of the new naming system the designation SE-400.
The radio station SE-508, which was introduced with the purchase of the French tank AMX, is technically almost identical. In the SE-408, however, the frequency range is significantly lower with its 20 - 27.9 MHz, in the upper shortwave range.
The radio station SE-400 or the corresponding original U.S. variant SCR-608 consists of two receivers BC-683-A and a transmitter BC-684-A mounted on a mounting frame FT-237. By default, the device is operated with 12V DC from the vehicle battery, a dynamotor generates the necessary plate voltages.
The device covers 10 crystal controlled channels in the range of 27 - 38.9 MHz. In a variant set, the frequency doubler stage was wired before the frequency tripler stage, so the twenty channels came in the 20 - 27.9 MHz frequency range; in Switzerland, this set got the designation SE-408.
The receiver has a large built-in switchable speaker, the VOLUME knob which is located almost in the middle of the front panel. The ten channel buttons on the top right are used to select the operating frequencies. By pressing the small PUSH to TUNE button on the left of the dial window, the receiver can also be continuously tuned using the knurled wheel below the station buttons.
In a bottom row of controls, the (auxiliary) antenna and ground connectors, the switch to tune and operate the set, the switch of the onboard intercom in a tank and the squelch control are located; the squelch threshold is set with the SENSITIVITY control.
On the Transmitter front panel, the controls are located on the right in a frame-protected area. On its left side, the ten fixed frequency buttons are located below the connectors for the (auxiliary) antenna and ground. The quartz crystals are inserted in the crystal holders located behind the cover on the left. The switch below the channel buttons switches from tune to operation.
At the right below the instrument, which can be set to tune and display the antenna current, the main switch and the switch for transceive - onboard intercom operation are located.
Receiver BC-683-A: The RF signal from the antenna is amplified (V1, 6AC7) and mixed with the signal of the RF oscillator (V3, 6J5) in the mixer stage (V2, 6AC7) to generate the intermediate frequency of 2.65 MHz. The IF is amplified in two IF amplifier stages (V4 and V5, two 12SG7) and has to pass a limiter stage (V6, 6AC7). The demodulation is done in the discriminator stage (V7, 6H6), the AF signal is amplified in the AF preamp valve (V10, 6SL7GT), which also acts as IF oscillator) and the AF output stage (V8, 6V6GT). Tube V9 (6SL7GT) acts as AVC and squelch.
Transmitter BC-684-A: With the pushbutton unit, the corresponding transmitter crystal (operation frequency is 72 times the crystal frequency) is connected to the transmitting oscillator (V107, 1619). After an RF amplifier stage (V101, 1619), the RF signal is amplified along with the audio signal, coming from the microphone and amplified in two AF amplifier stages (V105, V106, two 1619), and then fed to the modulator coil. The signal rich of harmonics is rectified (V102, 1619) and its twelfth harmonic is filtered out. After a frequency tripler stage (V108, 1619) followed by a frequency doubler stage (V103, 1619), the transmitter output stage tube (V104, 1624) is driven. The operations frequency corresponds to 72 times the transmitter crystal frequency and the output power reaches 20 watts. The signal is fed to the antenna output socket over one of ten pretuned antenna circuits selected by the pushbutton unit.
Receiver BC-683-A: V1 (6AC7, RF amplifier); V2 (6AC7, (mixer); V3 (6J5, RF oscillator); V4 and V5 (12SG7, first and second IF amplifiers); V6 (6AC7, limiter); V7 (6H6, discriminator); V8 (6V6GT, AF final stage); V9 (6SL7GT, AVC, Squelch); V10 (6SL7GT, AF pre amplifier and IF oscillator); Dynamotor DM-34.
Transmitter BC-684-A: V101 (1619, 1st RF amplifier stage); V102 (1619, rectifier); V103 (1619, frequency doubler stage); V104 (1624, transmitter final amplifier); V105 (1619, 1st AF amplifier stage); V106 (1619, 2nd AF amplifier stage); V107 (1619, transmitter oscillator); V108 (1619, frequency tripler stage); Dynamotor DM-35.
In a joint development project, the Link Radio Corporation, supported by Western Electric and Bell Laboratories, developed a frequency-modulated VHF transceiver which was ready for production in 1940/41. Due to the use of frequency modulation, the set was less prone to sparks and soon became the standard vehicle set of the American armored corps. With a modified frequency range, the device was adapted to the needs of the artillery in 1943/44.
After the end of the Second World War, many of these sets were for sale in the surplus market. After successful trials, the Swiss Army acquired a number of the American VHF radio, which gave the impetus for the shift of the tactical military communication into the VHF frequency band.
The parallel development SCR-508 working on the slightly lower frequency band was also introduced as SE-408.
From surplus stocks, a total of 267 units of the SCR-608 were acquired in 1948/9, they were introduced as „Radio Station FIX“ or „SE-400“ as command set.
One major problem was the fact, that frequency-modulated devices could communicate with each other but not with the amplitude-modulated sets from the older generation of wireless equipment, so at times network planning with dual frequency planning was necessary.
The purchase of the SE-400 also anticipated the decision to use the VHF range and frequency modulation for military communications, which led to the redevelopment of the first Swiss FM radio generation SE-206/209 and SE-407, after the stock of American surplus equipment was exhausted.
The devices remained in troop service until 1970 and were liquidated in 1971.